Error when trying to create New Page

Hello,
I was recently asked by a friend to help him with his outdated WP installation.
After update of the theme of the website and all the plugins used, when trying to add a New Page, a blank page with the following message is displayed:

There has been a critical error on this website

Please help me with some advise on to where to start with debugging the issue.

The problem is with the qTranslate-X plugin that is installed, because when disabled the problem with New Page is not present, but if disabled the whole site is not working at all!

The theme in use is BeTheme.

Thanks & Regards,
Nikolay

The page I need help with: [log in to see the link]